Particularly when processing materials which are sensitive to shear and dwell time, for example commercial plastics or food compounds, on screw-type machines, that is particularly extruders, it is often necessary to influence the plasticizing or melting of these materials or the temperature thereof and other parameters by a throttling action at the end of the machine before the plasticized or molten materials are discharged. Furthermore, it is necessary to some extent to provide so-called starting valves which enable the materials to be channeled out of the machine during the starting operation until the plasticizing or melting process is set in such a manner that the material can be fed to the standard discharge element, that is for example an extrusion die, where it is brought into any form, for example for granulating. The normally used tandem arrangement of the throttle on one side and starting valve on the other side results in relatively long material flow paths in the discharge elements, which is unacceptable particularly in the case of materials sensitive to dwell time because the material can suffer thermal damage. In addition, deposits of the material on the discharge channel walls can occur.
A screw-type machine is known from German published patent application No. 21 21 305, in which discharge channel a throttle body is arranged, which throttle body can be actuated via a drive arrangement at right angles to the conveying direction. A starting valve is not provided in this case.
A screw-type machine having a so-called vane-type throttle is known from German patent specification No. 28 33 057 (corresponding to U.S. patent application Ser. No. 819,227), which throttle is arranged in a double screw bore and can be pivoted by pivoting movements about its longitudinal axis into positions having a variable throttling effect.
German published patent application No. 19 37 862 discloses a screw-type machine with a starting valve arranged in the discharge channel which allows the molten material alternately to be drawn off through a starting outlet opening or fed to the discharge element, for example an extrusion die. A throttle is not provided in this case.